Carbon Turnover in a Sorghum-cultivated Semi-arid Mediterranean Soil Profile

Organic carbon storage in soils is a key factor for agriculture sustainability and future climate control. Nevertheless, this issue has been insufficiently studied in the semi-arid context of northern Africa. Our aim was to quantify carbon dynamics in a Tunisian Calcaric Cambisol profile, using the natural C-13 labeling offered by the continuous C4 crop forage sorghum. Our results show that in the topsoil 0-30 cm, the amount of crop-derived carbon accounted for 20% of the cumulated input from crops after 12 years, and half of it was retrieved as organic matter with particule size < 50 mu m. Such values could be model-predicted with no particular adaptation to the specific calcareous environment using the RothC model. However, soils were additionally characterized by a very high subsoil organic carbon stock (170 t C ha(-1) between 30 and 140 cm), and we demonstrated that crop rooting highly contributed to this subsoil organic matter. Indeed, an unusually large proportion of new crop-derived carbon (one third) was found below the upper 30 cm. Our results show an increase in carbon content in the deep layers of this soil under semi-arid Mediterranean climate.